The function, reliability and performance of semiconductor devices depend on the use of semiconductor materials and surfaces which are clean and uniform. Billions of dollars and countless man-hours have been spent developing, characterizing, and optimizing systems and processes for fabricating and processing semiconductor materials. A primary goal of this activity has been the fabrication of materials and surfaces that are extremely clean and that have properties that are uniform, or vary uniformly, across the entire wafer. In order to characterize and optimize these processes it is necessary to be able to inspect and measure surface or bulk cleanliness and uniformity. For real-time process control, it is necessary to be able to make many measurements across a surface at high speed, and to do so in a manner that does not damage or contaminate the semiconductor surface. It is also highly desirable to be able to detect or classify multiple different types of non-uniformities or contaminants.
Many different technologies and systems have been used to measure surface or bulk properties of semiconductors. Many of these systems are highly sensitive to specific bulk or surface characteristics, such as metallic contamination, but these systems are often slow, destructive, or make measurements at only a few points. These systems may also be limited in the types of measurements they can make or defects that they can detect. For example, a system which detects metal contamination may not be able to detect organic contamination, a system which can detect particles may not be able to detect sub-monolayer contaminants, or a system capable of making precise measurements at one or more points on the wafer may not be fast enough to measure all points on a wafer at production speeds.
One known method of measuring or characterizing the condition of a surface is the vibrating Kelvin probe, sometimes called the Kelvin-Zisman probe. The Kelvin probe is a sensor that measures Contact Potential Difference (CPD). CPD is the difference in work function, or surface potential, of two conductive materials which are electrically connected. The Kelvin sensor consists of a conductive probe which is electrically connected to the surface to be measured. The probe is positioned close to the surface so that a capacitor is formed between the probe tip and the surface. A potential difference (voltage) results from the CPD between the probe tip and the surface. The probe tip is positioned at a point above the surface and then vibrated perpendicular to the surface so that the capacitance between the probe tip and the surface varies with time. This varying capacitance results in a time-varying current into the probe tip which is proportional to the voltage between the probe tip and the surface. This current is amplified to facilitate detection, and a variable bias voltage, sometimes called a backing voltage, is applied to the probe such that the time varying current goes to zero. When the current is zero, the bias voltage is equal and opposite to the CPD, so the CPD is determined. Many variations of the Kelvin probe have been developed. These include the Monroe probe, which vibrates a shutter in front of the probe tip instead of vibrating the tip itself; and scanning probes which make vibrating measurements at a series of points across a surface by stepping from one point to the next or moving slowly while the probe is vibrated. For relatively high-speed scanning, the probe can be operated with a fixed, or no, bias voltage and the magnitude of the probe current can be calibrated and converted to surface potential values. In all cases the signal is generated by varying the capacitance between the probe tip and the surface using vibration.
Kelvin probes are very useful in the characterization of many surfaces, including semiconductor surfaces. The Kelvin probe is useful because the work function of a surface, and resulting surface potential and CPD, are very sensitive to a wide range of surface conditions that can affect semiconductor device quality; such as contamination, surface chemistry, atomic surface roughness and surface charging. However, the Kelvin probe is essentially a point measurement technique. Although multiple measurements can be made at different points on a surface, or a series of adjacent points can be measured in series, it is difficult to measure more than a few points per second. Generating high resolution images of whole semiconductor wafers is a slow and time consuming process that is not well-suited to real-time process control applications.
A second method of characterizing a semiconductor utilizes Surface Photo Voltage (SPV). The electrical potential of a semiconductor surface is often sensitive to illumination with specific frequencies of light. A semiconductor surface, or an interface between a semiconductor and another material, will typically result in surface or interface-specific electron energy states. These states can cause surface charging and the formation of electric fields near the surface. This phenomenon of changing electrical potential near a semiconductor surface is known as band bending. Illumination of the semiconductor surface with super-bandgap wavelengths of light, and the subsequent generation, drift and recombination of carriers, act to reduce the level of band bending. Illumination of the semiconductor surface with sub-bandgap illumination can cause the population and depopulation of surface states that will also affect surface charging, band bending and the resulting surface potential. A variety of SPV-based tools have been developed to make a wide range of measurements on semiconductors and dielectric films on top of semiconductors. For example, SPV measurements can be used to detect doping densities, characterize the degree of band bending or determine the density and position of electron energy states at semiconductor surfaces and interfaces. These systems sometimes include the ability to apply controlled amounts of charge to the surface of a dielectric film. While SPV systems come in a variety of configurations with a range of measurement capabilities, these systems are all similar in that they make measurements by either 1) applying charge or illumination to the surface and then measuring the resulting surface potential or change in surface potential using a vibrating Kelvin probe, or 2) positioning a stationary capacitive probe over the surface and varying the charge or illumination to generate a time-varying signal that can be detected by the capacitive sensor. In other words, these systems generate a signal by varying the probe-to-surface capacitance, the illumination intensity, or the charge on the surface. Like the Kelvin probe, SPV measurement systems are essentially point measurement systems, and are not suitable for the generation of high resolution, whole wafer images at production speeds.
A third type of system for inspecting and measuring surfaces utilizes a non-vibrating contact potential difference sensor. Like the vibrating Kelvin probe, the non-vibrating contact potential difference sensor consists of a conductive probe that is electrically connected to the semiconductor surface. The probe tip is positioned close to the surface to form a capacitor, and a potential difference is formed between the probe tip and the surface due to the difference in work functions or surface potentials. Unlike the Kelvin probe, however, the non-vibrating contact potential difference sensor does not vibrate perpendicular to the surface. Instead, the probe tip is translated parallel to the surface, or the surface is translated beneath the probe. Changes in the work function or surface potential at different points on the surface result in changes in potential between the surface and the probe tip. This causes a current to flow into the probe tip. This current is amplified and sampled to form a continuous stream of data that represents changes in potential across the surface. The non-vibrating contact potential difference sensor can acquire surface data at a much higher rate than the vibrating Kelvin probe because the signal is not formed by vibration of the probe, but is instead formed by the relative scanning motion between the probe and the surface. The non-vibrating contact potential difference sensor can provide a continuous stream of data at rates greater than 100,000 samples per second. High data acquisition rates permit high-resolution whole wafer images to be acquired in only a few minutes.
While the non-vibrating contact potential difference sensor is well-suited to high-speed imaging of wafer surface potential, it produces data on only two wafer surface characteristics—changes in surface potential and changes in surface height. For semiconductor inspection applications, the sensor is usually operated to minimize the height signal by minimizing variations in the height of the probe above the wafer surface or minimizing the average potential between the probe tip and wafer surface. As a result, the non-vibrating contact potential difference sensor typically produces data on one characteristic of a surface—changes in surface potential.
It would be desirable to expand the capabilities of the non-vibrating contact potential sensor so that it could measure additional wafer characteristics and discriminate between different types of non-uniformities. For example, doping uniformity is an important characteristic of a semiconductor which affects many fundamental and critical semiconductor characteristics. However, it is difficult to identify doping density variations using the contact potential difference signal because the effect of doping density on work function is reduced or altered by surface or interface states that induce band bending near the wafer surface. Also, doping density variations may be difficult to separate from other non-uniformities such as variations in surface chemistry and contamination. It would be useful to expand the capabilities of the non-vibrating contact potential difference sensor so that it could detect additional semiconductor characteristics, such as changes in doping density, and distinguish between different surface and bulk non-uniformities. In addition, it would be desirable to improve the sensitivity of the non-vibrating contact potential difference sensor so that it could detect smaller or more subtle non-uniformities.